| Summary: | Automotive turbochargers (TCs) increase internal combustion engine power and
efficiency in passenger and commercial vehicles. TC rotors are usually supported on
floating ring bearings (FRBs) or semi-floating ring bearings (SFRBs), both of which are
inexpensive to manufacture. However, fluid film bearings are highly nonlinear
components of TC units and contribute to the complex behavior (i.e. bifurcations and
frequency jumps between a first whirl frequency and a second whirl frequency) of the
entire rotor-bearing system (RBS). The frequency jump phenomenon concerns the TC
manufacturing industry due to increased levels of noise generation. This thesis presents
progress on assessing the effects of some bearing parameters and operating conditions
on the RBS dynamic forced performance and the frequency jump phenomenon.
A fluid film bearing model is integrated into a finite element rotordynamics
computational model for numerical prediction of the TC linear and nonlinear (time
transient) forced response. Since automotive TCs operate with variable rotational speed,
predictions are conducted with shaft acceleration/deceleration.
Over most of its operating speed range, TC rotor nonlinear response predictions
display two subsynchronous whirl frequencies w1 and w
2 representing a conical mode
and a cylindrical bending mode, respectively. At low shaft speeds w1 is present up to a
shaft speed (Omega bifurcation), where there is a frequency jump from w1 to w
2. The second whirl
frequency may persist up to the highest shaft speeds (depending on operating
conditions). Results show during rotor deceleration the Omega bifurcation may be different from
the one during rotor acceleration (hysteresis).
Predictions show the following factors delay the Omega bifurcation: increasing oil supply
pressure, decreasing oil supply temperature, and increasing shaft acceleration. Also, rotor imbalance distribution greatly affects Omega bifurcation and the shaft amplitude of total
motion.
Overall, this study shows the sensitivity of bifurcations and frequency jump
phenomenon in TC nonlinear response due to various bearing parameters and operating
conditions. Further analysis is required to generalize these findings and to assess the
effect of other bearing parameters (i.e. clearances, outer film length, ring rotation, etc.)
on this phenomenon. In addition further validation of the predictions against test data is
required for refinement of the predictive tool.
|
|---|
